<p>
Choose the input elements (drag them here):
</p>
<center>
    <div class="elementDrag">a
    </div>
    <div class="elementSeparator">
        +</div>
    <div class="elementDrag">n
    </div>
</center>
<p>
Choose the resulting element (drag an element here):
</p>
<center>
    <div class="elementDrag">
    </div>
</center>
